Description

This article explains one of the reasons why the number of active BGP prefixes can be different between 'show bgp summary' and 'show route summary' outputs.

Symptoms

There are cases when the router can be configured with two or more route-reflector neighbors and receiving identical update messages from both the RRs with the same nexthop and other information associated with the route. It can be done for redundancy purposes and the following mismatch can occur:

  1. The output of 'show route summary' shows that only half of the routes are in active state while the rest of the routes are not installed in the routing table: 

    labroot@lab-re0> show route summary
Autonomous system number: 65500
Router ID: 10.16.53.9
inet.0: 119 destinations, 220 routes (119 active, 0 holddown, 0 hidden)
              Direct:      8 routes,      8 active
               Local:      5 routes,      5 active
                 BGP:    200 routes,    100 active 
....

  2. However, the output of 'show bgp summary' reveals that the number of active paths are equal to the number of total paths: 

    labroot@salo-re0> show bgp summary
Groups: 3 Peers: 8 Down peers: 6
Table          Tot Paths  Act Paths Suppressed    History Damp State    Pending
inet.0
               200        200          0          0          0          0 
    inet6.0
                 0          0          0          0          0          0
Peer                     AS      InPkt     OutPkt    OutQ   Flaps Last Up/Dwn State|#Active/Received/Accepted/Damped...
10.100.1.2              65500        3          3       0       3          16 Establ
  inet.0: 100/100/100/0
10.200.1.2              65500        4          3       0       3          12 Establ
  inet.0: 100/100/100/0

Solution

The output we are seeing from 'show bgp summary' output may seem strange because half of the routes which contain exactly the same nexthops should not appear on the active paths row, as in 'show route summary' output. However, this is the expected behavior and  is functioning as designed. 

In this case, the router is receiving identical update messages from both the RRs with the same nexthop and other information associated with the route. In order to reduce the multipath calculation job (to decrease CPU utilization), the following is checked:

  • If this is a duplicate route (i.e if the new route nexthop is already part of the multipath next hop), the multipath computation is skipped and this route is marked as a Multipath Contributor.

Whenever a route is considered as a Multipath Contributor, from BGP’s perspective, that route’s count is considered in the active prefixes count. This has been the behavior since day 1 and this is the reason multi path contributor routes are considered, while maintaining the active prefixes count in BGP statistics. Hence, the counts seen in 'show route summary' and in 'show bgp summary' do not match because they have come from different methods of calculation.
 

Modification History

2018-06-09: Minor sentence clarification. No technical changes.

Related Information